Direct reduction plants have been with an increasing share of metallic iron produced in the steel industry over the last decades. The production capacity of DRI (direct reduced iron) of the first plants, measured as metric tons of DRI per year, was in the range of 500,000 to 800,000 and since then direct reduction reactors have been improved in design and now operate producing 1,000,000, 1,600,000, 2000,000 and 2,500,000 or more tons of DRI per year.
Technical limitations in this scale-up of reduction reactors have been overcome, mainly related to achieving the downward flow of solids particles through the reactor at the proper rate and solids flow pattern so as to assure a uniform quality of the product which is closely related to improved reducing gas distribution, and the reactors can be properly designed for large production capacities, as mentioned before, without proportionally increasing the capacity of the typically associated reformer.
Reducing gases, mainly comprising H2 and CO can be supplied from a number of sources and conversion processes, for example from natural gas which is reformed by reaction with CO2 and/or H2O in a catalytic reformer; from partial combustion processes of gaseous, liquid or solid hydrocarbons such as natural gas, oil derivatives, fuel oil or coal; from pyrolysis of coal in coke ovens (coke oven gas), or from synthesis gas obtained from coal gasification.
It has been known that the hydrocarbon (for example natural gas) utilized in a direct reduction process is used for three purposes: (1) as a source of reducing gases (H2 and CO) which transformation is done by catalytic reforming of such hydrocarbon with oxidants (CO2 and H2O); (2) as fuel for generating heat for supplying the energy for reformation and/or for heating the reducing gas to the required temperature; and (3) as carburizing agent for increasing the carbon content in the DRI to the desired levels thus reducing the electricity consumption in the subsequent melting step of the steelmaking process.
The reducing gases (H2 and CO) are usually produced through the following reactions in the catalytic reformer:CH4+CO2→2CO+2H2 CH4+H2O→CO+3H2 
These reactions are endothermic and the energy is provided by the combustion of a suitable fuel, typically natural gas, supplemented by reducing gas purged from the system, and/or other available fuel.
The reduction of iron oxides is carried out through the following reactions:Fe2O3+3H2→2Fe+3H2OFe2O3+3CO→2Fe+3CO2 
Carbon content in the DRI is carried out mainly by the cracking reactions of hydrocarbons, (illustrated by the reaction of methane as the main constituent of natural gas):3Fe+CH4→Fe3C+2H2 
The energy necessary for driving the reforming reactions is supplied to the reformer by burning any suitable fuel; which can be, for example, natural gas.
The carbon content in the DRI is mostly obtained from the cracking of hydrocarbons, and to a lesser extent from the CO content of the reducing gas fed to the reduction reactor. The potential of CO to carburize the DRI in the reduction zone according to the reaction 2CO→C+CO2 is very low because at the inlet of the reduction zone the temperature is too high for the reaction to proceed and at the top of the reduction zone, where the temperature is favorable for the reaction to occur, there is no metallic iron serving as a catalyst for the reaction. Carburization by cracking of hydrocarbons is favored at high temperatures and is also catalyzed by metallic iron. These two factors are present at the bottom part of the reduction zone, where the hot reducing gas is introduced into said reduction zone, but a high concentration of hydrocarbons is necessary.
Hydrocarbon concentration in the reducing gases however cannot be high enough for producing DRI with a desired level of carbon because the gas effluent from the reformer has a low CH4 concentration after having reacted with oxidants (H2O and CO2) to produce H2 and CO. The amount of carbon in the DRI produced in the plants where off-gas from the reactor is recycled through an in-line reformer is typically between 1.5% and 2% by weight, while in those plants having a reformer outside of the recycled gas circuit; and an independent recycle circuit through a gas heater, the carbon content in the DRI may be from 1% to about 4% by weight.
The present invention is addressed to a method and apparatus for producing DRI having a controlled amount of carbon by controlling the amounts of hydrocarbons and CO of the gases within the reduction reactor.
Applicants have found the following patents and patent applications related to the recirculation of top gas from a reduction reactor through a hydrocarbon reformer and a separate gas heater:
U.S. Pat. No. 6,395,055 to Bueno et al. discloses a direct reduction process wherein top gas effluent from the reduction reactor can be recycled in two ways, one through a catalytic reformer and other through a gas heater. Make-up natural gas can be fed to the top gas stream fed to the reformer and also to the top gas stream fed to the heater. This patent however does not teach or suggest to remove CO2 from the portion of the top gas that is recycled through the gas heater, and therefore the amount of top gas that can be recycled is relatively low because a high amount of top gas must be purged and used as fuels in order to get rid of the carbon (as CO2) fed as methane and other hydrocarbons in the natural gas make-up. Although this patent shows that CH4 may be added directly to the reactor it entails the disadvantage of requiring an excessive amount of oxygen for supplementing the heat necessary to raise the temperature of the reducing gas fed to the reactor.
U.S. Pat. No. 6,027,545 to Villarreal discloses a direct reduction process with an improved reducing gas utilization wherein a portion of the reducing gas effluent from the reactor, which would normally be burned as fuel, is recycled to the reactor after regeneration of its reducing potential, by removing water and carbon dioxide therefrom. One embodiment of this patent illustrated in FIG. 3 comprises recycling a portion of the top gas of the reactor by re-heating it in a separate gas heater 34A after CO2 is removed so that most of the top gas containing H2 and CO can be recycled back to the reduction zone of the reactor thus utilizing as much as possible of the reducing gas.
Although, this patent teaches a significant improvement in the reducing gas utilization, there is however no teaching nor suggestion therein about having a first make-up amount of natural gas which will be reformed in the catalytic reformer and having also a second make-up natural gas stream fed with a second portion of CO2-lean top gas nor a teaching of a predetermined distribution of the make-up natural gas fed to the reformer and the make-up gas fed to the reduction zone for obtaining DRI having a predetermined amount of carbon.
U.S. Pat. No. 8,377,417 discloses a direct reduction process wherein CO2 produced in the reduction reactor is stripped from the top gas and can be sequestered for its disposal in a controlled manner instead of emitting it to the atmosphere as part of the fuel burned in the reformer. This patent suggests recycling a portion of the CO2-lean top gas to the reduction reactor which can optionally be preheated using the heat content of the flue gases produced in the reformer after some heat has been taken for steam production. The main make-up stream of natural gas is fed to the reformer where it is consumed; so consequently no control of carbon in the product can be obtained. There is a second natural gas stream added to the reducing gas fed to the reactor, but this addition is not effective to increase the hydrocarbon concentrations; because oxygen also added would have to compensate for the temperature drop caused by such natural gas addition, and therefore the hydrocarbons will be understood to react with said oxygen in order to reach the required reducing temperature. There is no teaching or suggestion in this patent of having a second make-up natural gas stream effective for the expressed purpose of increasing the hydrocarbon concentrations within the reduction reactor; and especially there lacks any teaching of the possibility of a predetermined distribution of the relative amounts of first and second make-up natural gas streams for obtaining a controlled amount of carbon in the DRI and/or a better energy efficiency of the direct reduction process.